Cleaning and activating compositions and use thereof in producing protective phosphate coatings on metal surfaces



Patented Dec; 6, 1949 CLEANING AND ACTIVATING COMPOSI- TIONS AND USE THEREOF IN PRODUC- lNG PROTECTIVE PHOSPHATE COATINGS 'ON METAL SURFACES George w. Jernstedt, Pittsburgh, Pa., Westinghouse Electric Corporation,

assignor to East Pittsburgh, Pa., a. corporation of Pennsylvania No Drawing. Application January 21, 1949, Serial No. 12,063

Claims.

This invention relates to the art of producing corrosion-resistant coatings on the surfaces of iron, zinc, cadmium, aluminum, and other metals and alloys. This invention relates more particularly to the production of protective phosphate coatings upon the surfaces of iron, zinc, cadmium, aluminum, and the like.

This application is a continuation-in-part of my copending patent application Serial No. 745,093, filed April 30, 1947, which in turn is a continuation-in-part of serial No. 510,251, filed November 13, 1943; both said applications are now abandoned, and each is entitled Protective coatings for metal surfaces."

It is an extensive practice at the present time to apply phosphate solutions to iron, zinc, cadmium, aluminum, and similar metal surfaces to react therewith to produce thereon an inherent protective phosphate coating. coatings provide for a high corrosion-resistance upon the surfaces of metals to which they are applied, and they are particularly desirable bases for the subsequent application of organic finishes thereon. Organic finishes will bond and adhere more tenaciously to the metal since the applied phosphate coatings are superior to the untreated bare metal surfaces as a base for such finishes.

In applying the protective phosphate coatings to the metal surface, it is desirable to secure the most expeditious formation of the protective phosphate coating and at the same time produce as complete and uniform coverage of the metal surfaces as can be reasonably attained. It has been previously discovered that the metal surface can be pretreated with certain compositions which will activate the metal surface so that it will react more rapidly and completely with solutions capable of producing protective phosphate coatings thereon and thus reduce the time required to secure .an adequate protective phosphate coating. By the preliminary use of such activating solutions, phosphate coatings have been produced on zinc surfaces in about a minute or less.

, According to prior practice, a complete process for treating sheets of iron, for example, involves, first, the cleaning of the sheets in an alkaline solution consisting, for instance, of soda ash, sodium silicate or sodium hydroxide. Secondly,

The phosphate spraying or flowing thereon,

the cleaned sheets are activated by applying thereto an activating solution. The activating solution consists of an aqueous solution having from 0.1% to 2% of disodium phosphate and from 0.001% to 0.1% of at least one of titanium, zirconium, lead, and tin radicals present in the form of compounds thereof. The time of application of the activating composition is relatively brief, 15 seconds usually being sufficient. Thereafter the sheets are treated to produce a protective phosphate coating thereon by applying as by or immersing in, a solution containing phosphates, phosphoric acid and an oxidizing agent. The treatment with this last-named solution may generally require a time period of from less than 30 seconds to a minute or more, depending on conditions.

It has been disclosed previously that water soluble titanium, zirconium, lead and tin compounds in combination with disodium orthophosphate are all suitable for activating iron, zinc and the like metal surfaces for expediting and improving the formation of phosphate coatings on such metal surfaces. Patents 2,310,239 and 2,329,065 and my copending patent application, serial No. 496,619, filed July 29, 1943, now 1'). S. Patent 2,462,196, disclose these activating compositions. They are now recognized as forming a class 01' elements for this purpose.

The object of this invention is to provide for producing protective coatings on metal surfaces by activation of the metal surfaces in an alkaline activating solution prior to treatment in a solution producing the protective phosphate coating.

A further object of the invention is to provide for simultaneously cleaning and activating metal surfaces prior to applying a protective phosphate coating.

A still further object is to provide stable dry compositions comprising alkali cleaners and an activating composition which when dissolved in water to provide a solution having a pH of between 10 and 12.5 will efiectively clean and activate metal surfaces for superior phosphate coatings reactively.

Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.

It has been discovered that compositions capable of activating the surfaces of iron, zinc, cadmium, aluminum, and the like may be combined vide forv a uniform,

3 with alkaline cleaning agents to provide for improved activation of the metal surface whereby better protective coatings are obtained as well as enabling a reduction in the time required for a complete treatment of the metal to produce such coatings.

According to this invention, the activating composition and the alkalinecleaner are combined for simultaneous application with greatly improved results whereby a better protective phosphate coating is ultimately secured. To this end, an alkaline cleaning and activating solution may be prepared by combining an alkali cleaner, for example, 4 or 5 ounces of soda ash which when added to a gallon ofwater will produce a solution having a pH of to 12.5 and admixing therewith from 0.13 to 2.6 ounces of a previously prepared activating composition composed of disodium phosphate combined with a minor proportion (20% or less) of a compound of titanium, zirconium, lead or tin in the manner described hereinafter. The disodium phosphate composition is added in an amount to produce a concentration of disodium phosphate of from 0.1 to 2% of the weight of the aqueous solution. The order of addition of the alkali cleaner and disodium phosphate activating composition to water is not material.

When sheets of steel, zinc, zinc-coated steel,

cadmium. or aluminum, are treated with the alkaline solution carrying the activating solution therein unexpectedly better activation of the metal surface is attained as compared to applying each separately. The alkaline cleaner apparently etches the metal surface slightly and concuractivating rently with the etching action the composition activates the metal surface to prohighly reactive state. Better activation is obtained in this way than if the activating composition were to be applied at a subsequent time independently of the cleaning operation.

--the alkaline cleaner is not I tions are effective in the cleaner and activating solution separately. The corrosion resistance of steel or zinc surfaces is improved appreciably since, due to the greater activation-ofthe metal surfaces, more complete and more uniform coverage of the steel surface by the phosphate crystals takes place. The crystals are much finer and provide for better paint adhesion when a paint or other organic finish is applied, subsequently to sheets treated in accordance with the present invention as compared to sheets treated by previous practice.

One reason for the superior results attained in the practice of the present invention resides in the fact that the steel, even after alkali cleaning, may have small scales retaining acid and other deleterious material that is released in the activatingcomposition, since the brief treatment in always sufficient to react with and completely eliminate such acid.

When combined with the alkaline cleaner, the presence of the alkali enables the maintenance of the aqueous solution at a pH of 10 to 12.5 which has been found to be most desirable for maintaining the activating composition at its highest eiliciency, while at this pH, the solution possesses excellent cleansing properties.

The alkaline cleaning composition may be any suitable alkali metal compounds particularly the hydroxides, carbonates, bicarbonates, borates, phosphates and silicates, which when placed in water solution individually or in admixture produce a pH of between 10 and 12.5, and such soluremovin'g surface dirt from metal surfaces. The term alkaline cleaning composition will be employed hereinafter to indicate such compounds. Examples of suitable alkali metal cleaning compositions are sodium carbonate or soda ash, sodium silicates such as sodium metasilicate, trisodium phosphate, borax,

By combining the alkaline cleaner with the activating composition, the total time required for treating the sheets to clean and to activate them of this invention produces cleaning and activation of the metal in a total time of about 15 seconds. This saving in time is quite advantageous and critical. A saving in processing equipment required is likewise achieved.

An example of the savings produced by the practice of the present invention may be observed in the phosphate coating of sheet steel as it is produced in a steel mill. The strip or sheet steel in the mill may be finish rolled at a rate of 600 feet a minute. Therefore, previously it would have required a tank of at least approximately 150 feet long for the cleaning and another tank 150 feet in length for the activation of the metal, in addition to duplication in rollers, drives and other auxiliary equipment. By employing a combined solution as taught herein, a single tank 150- practice of this phase sodium hydroxide, potassium carbonate and mixtures thereof. The alkali metal carbonates have given exception-ally good results. The amount of the alkaline cleaning composition in a given quantity of water is a factor in determining the pH of the solution and the user should use the proper amounts to provide a solution having a pH of between 10 and 12.5, or by admixing a stronger alkali with a weaker alkali, for example, a mixture of sodium hydroxide and sodium bicarbonate or sodium carbonate admixed with sodium bicarbonate.

The disodium orthophosphate in the proportions of from to pounds is preferably combined with from 20 to 0.005 pound respectively, the total weight being substantially 100 pounds, of one or more water soluble compounds of titanium, zirconium, lead or tin, by dissolving in water and evaporating the solution to dryness. The solid residue so produced possesses activating characteristics and may then be combinedwith the dry alkali cleaner or to an aqueous solution thereof. the activating composition and the alkaline cleaning composition, such as soda ash, in suitable proportions may be prepared and shipped or stored, and when required a weighed quantity dissolved in a given amount of water to provide a solution of the proper pH.

Examples of water soluble compounds of titanium, zirconium, tin and lead, that have been found satisfactory are titanium carbide, titanium tetrachloride, titanium trichloride, titanium hydroxide, titanium nitr de, titanium potassium oxalate, titanium dioxide, zirconium chloride, zirconium sulphate, lead acetate. tin tetrachloride and stannic sulphate. Since only relatively small amounts of any titanium, zirconium, lead and tin compound need be dissolved, the compounds selected need be only slightly soluble in water to be suitable. Since the weight proportion of the required metal present varies from compound to compound, the amount of compound employed should be selected accordingly.

As an example of the preparation of the activating composition, in about 150 pounds of water there were dissolved 100 pounds of disodium orthophosphate and 3 pounds titanium potassium oxalate. The solution was heated to about 75 C. and slowly evaporated to a dry residue. The residue was ready for use by dissolving in water.

A satisfactory alkaline cleaning and activating composition solution may be prepared by dissolving from 2 to 8 oz. of an alkaline cleaning composition per gallon of water, and from 0.13 to 2.6 ounces of the activating composition to provide a concentration of from 0.1% to 2% by weight of disodium orthophosphate and from 0.001% to 0.1% by weight of titanium, zirconium, tin or lead radical in the solution.

In installations consisting of separate cleaning and activating tanks, where removal of one of the tanks and adjusting of conveyors may be expensive and is not desirable, the benefits secured by adding alkali to the activating composition still may be found advantageous. Introducing into the separate activating composition a quantity of alkaline cleaning composition to provide for a pH of 10 to 12.5 will improve the activating composition. A particularly desirable component in the alkali cleaning agent for use in this combination has been found to be sodium carbonate. Since sodium hydroxide and sodium silicate in aqueous solution will be gradually converted to the carbonate by reason of contact with air, they will be found to be as satisfactory as the alkali carbonates after a short time. If sufficient sodium carbonate is added to the preactivating composition to attain a pH of 10 to 12.5, further cleaning and etching will take place in the activating tank in addition to that taking place in the preceding treatment in the regular alkali cleaner tank. The simultaneous etching and activating of the metal in this instance produces nearly the full equivalent benefit of that secured by the simultaneous cleaning and activating of the metal surfaces.

Both the alkali cleaner and disodium phosphate activating composition may be replenished from time to time as they are used up. Ordinarily the activating component and the alkali cleaner component are consumed or dissipated in about the same proportions as they are combined, and a mixture of both may be added to the solution.

The solution with the combined alkaline cleaning composition and activating composition is usable at any temperature from room temperature to boiling, but the effective life is progressively shorter as the temperature exceeds 140 F. Thus at temperatures of up to 120 F., the solutions may be kept for months without any substantial loss of their effectiveness, and at 140F. the solutions are usable even after several weeks, while at 200 F. the life is of the order of a few hours. Accordingly the recommended solution temperatures for industrial uses should not greatly exceed 140 F. if a reasonably long effective life is desired.

In carrying out the process by dipping metal into the bath, it is desirable to stir the treating bath thoroughly to uniformly distribute the ingredients. Where the bath is applied by spraying or the like, stirring will not be required.

After the metal surface has been treated with the alkali cleaner and activated, it may be sub- 5 jected to an appropriate phosphate coating Solution such, for example, as are listed in my prior Patent No. 2,310,239, issued February 9, 1943. Such phosphate coating, solutions usually comprise a dilute aqueous solution of a phosphate salt, 10 phosphoric acid and an oxidizing agent. For producing a phosphate coating on ferrous metal surfaces previously cleaned and activated with the compositions of this invention, to the ferrous metal there may be applied an aqueous solution containing Manganese phosphate 1% pounds Phosphoric acid-85% 5 pounds Sodium nitrate pound Cupric nitrate ,4; ounce Water to make 30 gallons In some cases, after the phosphate coating has been produced on the metal surface, a sealing treatment with chromic acid may be employed.

The sealing treatment normally comprises applying an aqueous solution, generally at temperatures of 170 F. to 200 F., containing 7 /2 ounces chromic acid in 100 gallons of water.

-. Since certain changes may be made in the above invention and different embodiments of the invention may be made without departing from the scope hereof, it is intended that all matter contained in the disclosure shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. An aqueous solution for treating the surface of a metal from the group consisting of ferrous, zinc, cadmium and aluminum metals to provide for simultaneously activating and cleaning them, consisting of a composition derived by dissolving in water 0.005% to 20% by weight of a water soluble compound of at least one of the group of metals consisting of titanium, zirconium, lead and tin and the balance being 5 disodium orthophosphate and evaporating the solution to dryness, the residue so derived being dissolved in water to provide a concentration of from 0.1% to 2% by weight of the disodium orthophosphate and a concentration of from 0.001% to 0.1% of the metal radical of said water soluble compound, an alkaline cleaning compound suitable for cleaning the metal surface to provide a. pH of from 10 to 12.5 in the aqueous solution, and the balance being water.

2. An aqueous solution for treating the surface of a metal selected from the group consisting of ferrous, zinc, cadmium and aluminum metals to provide for simultaneously activating and cleaning them, consisting of a composition derived by dissolving in water 0.005% to 20% by weight of a water soluble compound of at least one of the group of metals consisting of titanium, zirconium, lead and tin and the balance being disodium orthophosphate and evaporating the solution to dryness, the residue so derived being dissolved in water to provide a concentration of from 0.1% to 2% by weight of the disodium orthophosphate and a concentration of from 0.001% to 0.1% of the metal radical of said com- 7 pound, an alkali metal carbonate in an amount to provide a pH of from 10 to 12.5 in the aqueous solution, and the balance being water.

3. The process of simultaneously cleaning and activating the surfaces of metals selected from the group consisting of ferrous, zinc, cadmium 7 and aluminum metals to enable an improved subsequent formation of protective phosphate coat- 7 ings thereon comprising applying to the metal surface an aqueous solution consisting of a composition derived by dissolving in water 0.005% to 20% by weight of a water soluble compound of at least one of the group of metals consisting of titanium. zirconium, lead and tin and the balance being disodium orthophosphate and evaporating the solution to dryness, the residue so derived being dissolved in water to provide a concentration of from 0.1% to 2% by weight of the disodium orthophosphate, an alkaline cleaning composition suitable for cleaning the metal surface to provide a pH of from 10 to 12.5 in the aqueous solution,- and the balance being water, and then applying to the cleaned and activated metal surface a phosphate coating composition comprising a dilute aqueous solution con a phosphate salt, phosphoricacid and an oxidizing agent.

4. A composition suitable for cleaning and activating metal surfaces when dissolved in water composed of (a) from 2 to 8 parts by weight of an alkaline cleaning composition and (b) from 0.13 to 2.6 parts by weight of an activating composition derived by dissolving in water from 80 to 100 parts by weight of orthodisodium phosphate and from 20 to 0.005 parts by weight of at least one water soluble compound of a metal from the group consisting of titanium, zirconium, lead and tin, the total being 100 parts and evaporating the last-mentioned solution to dryness.

5. A composition suitable for cleaning and activating metal surfaces when dissolved in water composed of (a) from 4 to 5 parts by weight of sodium carbonate and (b) from 0.13 to 2.6 parts by weight of an activating composition derived by dissolving in water from 80 to 100 parts by weight of orthodisodium phosphate and from 20 to 0.005 parts by weight of at least one water soluble compound of a metal from the group consisting of titanium, zirconium, lead and tin, the total being 100 parts and evaporating the lastmentioned solution to dryness.

GEORGE W. JERNS'I'EDT.

No references cited. 

